Discontinuous carbon fiber reinforced metal matrix composite

ABSTRACT

Disclosed are methods and materials for preparing metal matrix composite (MMC) components that have low weight, good thermal conductivity and a controllable in-plane coefficient of thermal expansion. One embodiment of the invention features a metal matrix composite that includes a metal alloy and random in-plane discontinuous fibers. In some embodiments, the metal alloy includes aluminum, copper or magnesium. In certain embodiments, the metal matrix composite includes additives that enable solution hardening. In other embodiments, the metal matrix composite includes additives that enable precipitation hardening. Another embodiment of the invention features a method of manufacturing a metal matrix composite. The method includes contacting random in-plane discontinuous fibers with a binder, and pressurizing the random in-plane discontinuous fibers and the binder to form a bound preform. The preform is pressurized to a pressure greater than the molten metal capillary breakthrough pressure of the bound preform. Subsequently, the bound preform is placed in a mold, infiltrated with a molten infiltrant, and the molten infiltrant is cooled to form the metal matrix composite.

[0001] This invention was made with government support under Grant No.N00167-99-C-0072. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0002] The present invention relates to methods of forming metal matrixcomposites for thermal or structural applications, and the resultingcompositions. More specifically, the invention relates to methods ofinfiltration casting to form metal matrix composites with controlledthermal expansion and mechanical properties, and the resultingcompositions.

BACKGROUND OF THE INVENTION

[0003] Technological developments from cellular phones to imagingsatellites push current semiconductor device capabilities to theirperformance limits. In particular, modern devices often must dissipate agreat amount of heat. Integrated circuit devices typically requireintegration with heat sinks due to the potentially deleterious effectsof heat generated by the device. A semiconductor die, typically aportion of a silicon wafer, can be directly attached to a heat sink.More commonly, the die is encased in a ceramic package that protects thedie and provides electrical connections.

[0004] Common ceramic package materials include aluminum oxide, aluminumnitride and beryllium oxide. The coefficient of thermal expansion of thesemiconductor die and the ceramic package are purposely matched to avoidthermal cycle induced mechanical stress failures. Thermal cycling arisesduring power up and power down cycles in combination with resistiveheating due to current flow in the device.

[0005] Heat sinks are commonly fabricated from metals, for examplecopper, molybdenum, tungsten and aluminum. A metal heat sink is oftenplated with nickel prior to attachment to a ceramic package at anelevated temperature, for example, via brazing. Alternatively,silver-filled adhesives, or other conductive metal powder-filledadhesives, can be used for bonding.

[0006] Choosing a metal or other material for a heat sink often involvesa trade-off between desirable and undesirable properties. For example,aluminum and copper have high thermal conductivity, but coefficients ofthermal expansion several times greater than that of a ceramic packageor semiconductor die. During power cycling of the integrated circuit,resistive heating causes the temperature of the integrated circuit, andthe attached heat sink, to fluctuate. Consequently, such metals applymechanical stress to the heat sink bonding material during powercycling. The differential expansion of the heat sink relative to theceramic package or semiconductor die can cause failure of the bondmaterial or cracking of the package or die.

[0007] In contrast, some metals, such as tungsten and molybdenum, haverelatively small coefficients of thermal expansion. Although such metalscan permit a reliable bond, they have lower thermal conductivity thanaluminum or copper substrates and they are difficult to electroplate.Further, tungsten and molybdenum are undesirable for applications thatrequire minimal weight.

[0008] Composites of copper and tungsten, or of copper and molybdenum,can partially mitigate these deficiencies. These composites can be madeby powder metallurgical methods, such as infiltrating copper into asintered body of tungsten or molybdenum, or sintering a mixed powder ofthe two metals. It is difficult, however, to obtain an elongated plateby rolling a sintered ingot of tungsten or molybdenum. Alternatively,layers of metal can be joined by cladding or lamination of sheets.Cladded and laminated products require precise machining, which isdifficult and increases costs.

[0009] As an alternative to an all metal heat sink, some heat sinkscombine a sintered ceramic with a metal matrix. The fabrication processinvolves the formation of a ceramic preform, for example, by sinteringsilicon carbide powder. The ceramic preform microstructure typically hasa predetermined void volume fraction that is subsequently filled withmolten metal, typically aluminum. An aluminum ceramic heat sink canemploy copper-based inserts to improve its thermal conductivity. Suchheat sinks, however, can be difficult to machine and are usually limitedin their ability to match coefficients of thermal expansion withintegrated circuits.

[0010] As another alternative, a metal matrix composite can include aninorganic fiber material. Infiltration of fibers has its owndifficulties, for example, problems with fiber wetting and non-uniformfiber distribution. In addition, molten metal infiltration of fibersunder pressure can displace the fibers due to the fiber breakthroughpressure threshold. Further, it is often difficult to control fibervolume fraction, and thus to obtain a desired property of the composite.These factors have limited use of metal matrix fiber composites as heatsinks.

[0011] As the semiconductor industry continues to implement everincreasing semiconductor die sizes and transistor densities to permitenhanced integrated circuit complexity, the heat generated bystate-of-the-art integrated circuits also increases. Thus, the challengeof coping with resistive heating is expected to become an ever morecentral concern in integrated circuit design.

[0012] Beyond the electronics industry, precision motion and controlcomponents and other mechanical hardware must be light weight, stiff,and damp unwanted vibrations. In many instances, conventional materials,such as aluminum and copper, are unable to meet the performance demandsof many emerging technologies.

SUMMARY OF THE INVENTION

[0013] It has been discovered that a metal matrix composite (“MMC”) thatincludes random in-plane discontinuous carbon fibers and a method offorming an MMC from a pressure-formed preform can solve many problems ofprior art heat sinks. The invention can overcome numerous problems, suchas: fiber collapse during molten metal infiltration; coefficient ofthermal expansion (“CTE”) mismatch; heat sink weight; limits on range ofCTE values; difficulty in obtaining high fiber density; limits incontrol of fiber orientation; machinability of a heat sink; and/or heatsink cost. The invention addresses these problems through the use of oneor all of the following: preforms prepared with pressures greater thanthe breakthrough pressure used during metal infiltration; in-planeoriented fibers; short fibers; and carbon fibers.

[0014] Use of random in-plane discontinuous fibers permits a high fibervolume fraction in the MMC (“in-plane” as used herein is understood asthe X-Y plane, for example, the plane parallel to the bonded surface ofa heat sink). Further, by using in-plane oriented fibers, substantiallyall of the fibers can contribute to the control of the CTE in the X-Yplane. Though Z-direction CTE is not controlled by in-plane fibers, suchcontrol is generally unnecessary for heat sink applications because theintegrated circuit or other object is attached to an X-Y orientedsurface of the heat sink.

[0015] Use of these in-plane oriented fibers permits selection of a CTEover a wide range of values. A desired volume fraction of in-planeoriented fibers is selected to obtain a desired CTE. By orientingsubstantially all fibers in the X-Y plane, a very high fiber volumefraction can be obtained. This permits selection of volume fraction overa wide range and a corresponding ability to select a wide range of CTEvalues.

[0016] Carbon fibers, in particular graphite fibers, have excellentmechanical and thermal properties for use in heat sinks of theinvention. In combination with an aluminum or other light metal, suchheat sinks typically are easily machined, have excellent heatconductivity, and are lightweight. An aluminum and graphite fiber MMC ofthe invention thus realizes the advantages of aluminum—lightweight, easymachinability and good heat conduction—in combination with theadvantages of graphite fibers—high Young's Modulus, small to negativeCTE, high tensile strength, high thermal conductivity and strong dampingproperties.

[0017] The invention also solves the problem of non-uniform fiberdistribution within the MMC. A preform that includes fibers and a bindercan be prepared via application of a pressure in a preform mold that isgreater than the molten alloy breakthrough pressure for the preform. Thebinder maintains the compressed configuration of the fibers in thepreform while the preform is removed from the preform mold and placed ina metal infiltration mold. The metal infiltration mold can maintain thecompressed fiber configuration upon removal of the binder, if theinfiltration mold is sized and shaped to conform to the preform. Becausethe fiber configuration remains in its compressed state, it issubstantially undisturbed during infiltration of molten metal at themolten metal breakthrough pressure.

[0018] In a broad aspect, the invention features a metal matrixcomposite that includes a metal alloy and random in-plane discontinuousfibers. The random in-plane discontinuous fibers may be carbon, andpreferably are graphite. The fibers typically are milled, and preferablyare ball milled. In preferred embodiments, the metal alloy includesaluminum, copper or magnesium.

[0019] In one embodiment, the metal matrix composite has a volumefraction of random in-plane discontinuous fibers in a range ofapproximately 0.15 to approximately 0.6. In another embodiment, aminority of the random in-plane discontinuous fibers are oriented out ofplane by an angle greater than 10°. In a preferred embodiment, therandom in-plane discontinuous fibers are uniformly distributed withinthe metal matrix composite.

[0020] In certain embodiments, the metal matrix composite may include acomponent that enables solution hardening. In other embodiments, themetal matrix composite may include a component that enablesprecipitation hardening. In one preferred embodiment, the metal matrixcomposite includes aluminum, silicon and magnesium. In another preferredembodiment, the metal matrix composite includes copper, chromium andzirconium.

[0021] In another aspect, the invention provides a method ofmanufacturing a metal matrix composite. The method includes contactingrandom in-plane discontinuous fibers with a binder, and pressurizing therandom in-plane discontinuous fibers and the binder to form a boundpreform. In the later step, the random in-plane discontinuous fibers andthe binder are pressurized to a pressure greater than the capillarybreakthrough pressure of the bound preform. Subsequently, the boundpreform typically is placed in a mold, heated under a vacuum to removethe binder, then heated to above the metal liquidus and infiltrated witha molten infiltrant. The molten infiltrant is then cooled to form themetal matrix composite.

[0022] In another embodiment, the method includes placement of a secondbound preform adjacent to the bound preform in the mold prior toinfiltration with the molten infiltrant. A surface of the bound preformcontacts a surface of the second bound preform, and removal of thebinder prior to infiltration causes the contacted surfaces of the twopreforms to merge, creating one continuous, metal matrix composite.

[0023] The binder may be removed (called debindering) prior toinfiltration with a molten infiltrant, e.g., via evaporation.Alternatively, the binder may partially or completely remain in thepreform during infiltration. For example, a volatile component of abinder may be removed prior to infiltration, leaving a residue in thepreform.

[0024] Reference to the figures herein is intended to provide a betterunderstanding of the methods and apparatus of the invention but are notintended to limit the scope of the invention to the specificallydepicted embodiments. The drawings are not necessarily to scale,emphasis instead being placed upon illustrating the principles of theinvention. Like reference characters in the respective figures typicallyindicate corresponding parts.

[0025] It should be understood that the order of the steps of themethods of the invention is immaterial so long as the invention remainsoperable, i.e., e.g., a preform is made prior to infiltration of thepreform. Moreover, two or more steps may be conducted simultaneously.

[0026] The foregoing, and other features and advantages of theinvention, as well as the invention itself, will be more fullyunderstood from the description, drawings, and claims which follow.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027]FIG. 1 is a graph that shows the variations of CTE with in-planevolume fraction for a theoretical model and for experimental embodimentsof aluminum and copper matrix composites of the invention.

[0028]FIGS. 2a and 2 b are scanning electron micrographs of anexperimental embodiment of a metal matrix composite having an aluminummatrix and graphite fibers. FIG. 2a shows a cross-section through theX-Y plane. FIG. 2b shows a cross-section through the X-Z plane.

[0029]FIGS. 3a-3 e illustrate formation of a preform according to anembodiment of the invention. FIG. 3a is a cross-sectional illustrationof dispensing fibers and binder into a preform mold base portion. FIG.3b shows the fibers and binder residing in the preform mold baseportion. FIG. 3c shows the fibers and binder under compression in thepreform mold. FIG. 3d shows binding of the fibers by the binder. FIG. 3eshows a completed, bound preform after removal from the preform mold.

[0030]FIG. 4 illustrates-an embodiment of a stacked preform having threeindividual preforms layered with two graphite foils between thepreforms.

[0031]FIGS. 5a and 5 b illustrate an embodiment of forming a largerpreform from a combination of smaller preforms. FIG. 3a shows a stack ofpreforms without any intermediate layers. FIG. 3b shows the largerpreform after merging of the interfaces of the smaller preforms.

[0032]FIG. 6 illustrates an embodiment of the stacked preform of FIG. 4in a metal infiltration mold.

[0033]FIG. 7 illustrates an embodiment of a horizontally orientedpreform in a metal infiltration mold.

DETAILED DESCRIPTION OF THE INVENTION

[0034] A metal matrix composite (“MMC”) that includes random in-planediscontinuous carbon fibers and a method of forming an MMC from apressure-formed preform can solve many problems of prior art heat sinks.The composite and method can alleviate such problems as: fiber collapseduring molten metal infiltration; coefficient of thermal expansion(“CTE”) mismatch; heat sink weight; limits on range of CTE values;difficulty in obtaining high fiber density; limits in control of fiberorientation; machinability of a heat sink; and/or heat sink cost. Thefollowing describes various embodiments of the invention that mayinclude, for example, preforms prepared with pressures greater than thebreakthrough pressure used during metal infiltration, random in-planeoriented fibers, short or discontinuous fibers, and carbon fibers.

[0035] Definitions

[0036] As used herein, “molten metal infiltration” is understood to meanany casting process with or without an externally applied pressure tofacilitate infiltration of a mold vessel cavity that contains a preform.Examples of pressure infiltration casting include, but are not limitedto, pressure infiltration casting such as the Advanced PressureInfiltration Casting (APIC™) process as described in U.S. Pat. Nos.5,322,109; 5,553,658; and 5,983,973; high throughput pressureinfiltration casting as described in U.S. Pat. No. 6,148,899; squeezecasting; and die-casting.

[0037] As used herein, “metal” is understood to mean a metal or metalalloy. Examples of common metals or metal alloys are, among others,aluminum, aluminum alloys, bronze, beryllium, beryllium alloys,chromium, chromium alloys, cobalt, cobalt alloys, copper, copper alloys,gold, iron, iron alloys, steel, magnesium, magnesium alloys, nickel,nickel alloys, lead, lead alloys, copper, tin, tin alloys such astin-bismuth and tin-lead, zinc, zinc alloys, superalloys such asInternational Nickel 100 (IN-100) or International Nickel 718 (IN-718),and combinations thereof.

[0038] As used herein, “molten infiltrant”, “liquid infiltrant,” “moltenmetal,” or “liquid metal” is understood to mean a respective materialwhich is at least at or above approximately its liquidus temperature.

[0039] As used herein, “fugitive” is understood to mean substantiallyremovable, i.e., removable to a great extent.

[0040] As used herein, “preform” is understood to mean a fibrous,non-metallic material such as, e.g., an oxide, a boride, a nitride, acarbide or a form of carbon which is to be infiltrated with aninfiltrant. Infiltration of a preform by a molten metal followed bysolidification produces a metal matrix composite (MMC).

[0041] As used herein, “bound preform” is understood to mean a preformin which the fibers are held in a more or less fixed physicalrelationship due to the action of a binder material.

[0042] As used herein, “preform mold vessel” and “preform mold” areunderstood to mean any container capable of holding or applying pressureto preform materials during formation of a preform.

[0043] As used herein, “metal infiltration mold vessel” and “metalinfiltration mold” are understood to mean any container capable ofholding a preform, and confining the preform and molten metal duringmetal infiltration of the preform.

[0044] As used herein, “in-plane” is understood to mean the X-Y plane orthe plane normal to the Z direction in an X-Y-Z coordinate system. It isalso understood to mean the plane that is parallel to the bonded surfaceof a heat sink. This is commonly referred to as the “base” plane in theelectronics industry.

[0045] Overview of Materials in an MMC Component

[0046] Some embodiments of MMC components of the invention are wellsuited as heat sinks for use with a variety of integrated circuitsemiconductor and ceramic packaging materials. These components haverelatively low density, high thermal conductivity and a coefficient ofthermal expansion (“CTE”) that can be controlled over a wide range tomatch a companion integrated circuit material. Properties of commonsemiconductor and packaging materials are illustrated in Table I. TableI also shows the preferred heat sink CTE ranges for a good match witheach of the listed materials. TABLE I Semiconductor CTE PreferredDensity or Ceramic (ppm/° K.) Heat Sink CTE (g/cc) Si 4.2 4.5-5.0 2.3GaAs 6.5 7.0-8.0 5.3 AlN 4.5 5.0-6.0 3.26 Al₂O₃ 6.5 7.0-8.0 3.6 BeO 7.68.0-9.0 2.9

[0047] In one embodiment, an MMC includes random in-plane discontinuousfibers. Use of discontinuous fibers, particularly fibers less thanapproximately 1 mm in length, permits good control of the volumefraction of the fibers in the finished MMC. Further, for in-planeoriented fibers, substantially all of the fibers contribute to controlof CTE in the X-Y plane.

[0048] An MMC of the invention may include fibers of a narrow or widerange of fiber lengths. In some embodiments, an MMC includes fibers asshort as approximately 30 μm. In some embodiments, an MMC includeschopped fibers as long as approximately 12 mm.

[0049] Some applications do not require control of Z-direction CTE, forexample, heat sinks. For these applications, MMC components can befabricated with a desired X-Y plane CTE over a very wide range byselection of a corresponding fiber volume fraction. Hence, the inventionprovides heat sinks that can be CTE-matched to a wide variety ofintegrated circuit materials.

[0050] Fibers of various materials may be used. For example, the fibersmay include silicon carbide. In preferred embodiments, MMC componentsinclude carbon fibers, preferably graphite fibers, in a light metalmatrix. Carbon fibers may, e.g., be prepared from a pitch or panprecursor. Preferred embodiments employ pitch precursor fibers due to asuperior elastic modulus and thermal conductivity relative to panprecursor fibers.

[0051] Graphite fibers have excellent mechanical and thermal propertiesfor use in heat sinks of the invention. In combination with a lightmetal, e.g., aluminum, magnesium, or copper, or their alloys, such heatsinks are easily machined, have excellent heat conductivity and are verylightweight. An aluminum and graphite fiber MMC of the invention thusrealizes the advantages of aluminum—lightweight, easy machinability andgood heat conduction—in combination with the advantages of graphitefibers—high Young's Modulus, small to negative CTE, high tensilestrength, high thermal conductivity and strong damping properties.

[0052] In addition to high thermal conductivity, graphite fibers havethe unusual feature of a high modulus of elasticity combined with anegative coefficient of thermal expansion. Thus, these high modulus,negative CTE fibers can be embedded within a matrix metal alloy torestrain the matrix from expanding to its full extent during heating.The fibers further prevent excessive contraction during cooling from aprocessing temperature, and contribute to in-plane thermal conductivity.By combining graphite fibers with a metal such as Al and by controllingthe volume fraction of fibers and the orientation, one can design amaterial with a specified CTE.

[0053] In some embodiments, a heat sink of the invention can be solderedor brazed to an integrated circuit package to improve heat transfer. Theheat sink has a CTE that is preferably chosen to be slightly greater invalue than the package so that the package is under compression at roomor operating temperature. Package cracking or failure of the heatsink/package bond is less likely under this condition.

[0054] The CTE of the heat sink is also preferably chosen in view of thetemperature range to be used during attachment of the heat sink to aceramic package. For example, a eutectic copper-silicon braze alloyrequires a temperature of 780° C. during attachment of a nickel platedcopper alloy matrix heat sink to a metallized ceramic package. Agold-germanium braze alloy requires a temperature of 380° C. duringattachment of an aluminum matrix heat sink to a ceramic package.

[0055] The discontinuous fibers can be inorganic. Preferably, thediscontinuous fibers are carbon-based. The fibers are more preferablygraphite. The fibers can be chopped. In some embodiments the fibers areless than 25 mm in length. In some embodiments, the fibers arepreferably less than 1 mm in length, more preferably less than 0.75 mm.In some embodiments, the fibers are milled. In preferred embodiments,the fibers are ball milled. In a preferred embodiment, the fibersgraphite and have an average length of approximately 0.2 mm (200 μm) anda diameter of approximately 10 μm.

[0056] Fiber Orientation and Fiber Volume Fraction

[0057] Theoretical considerations can assist in the selection of a fibervolume fraction that will provide a desired in-plane CTE. For example, auniaxial laminate-based theory (see, e.g., R. S. Schapery, “ThermalExpansion Coefficients of Composite Materials Based on EnergyPrinciples,” J. Composite Materials, Vol. 2, pages 380-404, (1968))approximates the CTE of a composite material as: $\begin{matrix}{\alpha_{11} = \frac{\left. {{E_{f}\alpha_{f}v_{f}} + {E_{m}\alpha_{m({1 -}}v_{f}}} \right)}{{E_{f}v_{f}} + {E\left( {1 - v_{f}} \right)}}} & {{Equation}\quad 1}\end{matrix}$

[0058] where,

[0059] α₁₁ is the CTE of the composite in the orthogonal direction(parallel to the 0° fibers in an uniaxial laminate, units of ppm/°K),

[0060] α_(f) is the axial CTE of the fiber (units of ppm/°K),

[0061] α_(m) is the CTE of the matrix (e.g., 24 ppm/°K for Al and 16ppm/°K for Cu)

[0062] v_(f) is the volume fraction of fibers oriented parallel to the0° axis,

[0063] (1−v_(f)) is the matrix volume fraction,

[0064] E_(f) is the Young's modulus of elasticity for graphite fiber(units of GPa), and

[0065] E_(m) is the Young's modulus of elasticity for the Matrix (e.g.,69 GPa for Al and 110 GPa for Cu).

[0066] One can model orthogonal in-plane CTE properties, i.e., asobtained with two sets of orthogonally oriented in-plane fibers, byadding a second laminate of fibers oriented at 90° in the above model.Such a model should be considered a lower bound approximation fororthogonally oriented fiber reinforced metals.

[0067] One can approach the random in-plane condition by extension ofthe theoretical model to a multi-laminate composite. This will give anapproximate CTE for randomly oriented in-plane fibers where the X and Ycomponents of in-plane fibers are equal.

[0068] For most electronic thermal management applications when amatching CTE is important, it is desirable to maintain orthogonal CTEvalues, i.e., the CTE in the 0° or X direction should equal the CTE inthe 90° or Y direction. When the in-plane CTE is, for example, 8 ppm/°Kin all in-plane directions, one can simply say that the in-plane CTE is8 ppm/°K.

[0069] Balanced 0°-90° composites can be achieved by use of orthogonallyoriented in-plane fibers. Fibers in an MMC can be orthogonally orientedby weaving or by stacking alternating plies of uniaxially wrapped fibersto form a laminate of orthogonally oriented layers.

[0070] In preferred embodiments of an MMC, substantially uniformin-plane orthogonal properties are obtained by use of in-plane randomlyoriented discontinuous fibers. Preferably, very few of the fibers areoriented out of plane, i.e., in the Z axis direction. By controlling thevolume fraction of a preform, whether it includes woven, wrapped orrandom-in-plane discontinuous fibers, one can obtain a graphitereinforced MMC having a selected value of CTE.

[0071] Theoretical curves are plotted in FIG. 1 for an MMC that includesan aluminum matrix or a copper matrix, with fiber propertiescorresponding to those of P-120 graphite fibers, a fiber in theThermalGraph® family of products available from BP Amoco (Alpharetta,Ga.). These fibers have an average length and width of approximately 200μm and 10 ∞m, respectively, E_(f)=827 GPa, α_(f)=−1.45 ppm/°K, anddensity=2.17 g/cc. Other useful fibers in the ThermalGraph® familyinclude those sold under the developmental names DKA X and DKD X.Similar fibers are available from other suppliers, such as Conoco CarbonFibers (Houston, Tex.).

[0072] Theoretical curves can be used to assist control of the CTE of areinforced metal alloy by selecting an appropriate fiber volumefraction. For example, referring to FIG. 1, a CTE in a range of 4 to 12ppm/°K would require a corresponding fiber volume fraction betweenapproximately 0.40 and 0.18. Similarly, a P-120 reinforced copper alloyMMC would require selection of fiber volume fraction between 0.4 and0.14 to obtain CTE values in the same range.

[0073] More generally, MMC components can be prepared with a wide rangeof CTE values. For example, a CTE can be zero or negative in value, orcan be 12 ppm/°K or greater in value. The ability to densely pack fiberspermits fiber volume fractions to be chosen, preferably, from within arange of approximately 0.15 to 0.60. For some applications, fiber volumefraction may lie outside this range.

[0074] For aluminum-based MMCs, a wide range of useful properties may beobtained with a fiber volume fraction in a range at least as broad asapproximately 0.15 to approximately 0.55. For example, a fiber volumefraction of 0.30 provides a CTE of approximately 8.0 ppm/°K, while afiber volume fraction of 0.40 provides a CTE of 4.0 ppm/°K.

[0075] Deviation of a portion of the fibers in an MMC from in-planeorientation reduces the observed CTE from that predicted by theory.Deviations from in-plane randomness in experimental samples cause thein-plane CTE of a reinforced metal alloy to be greater than thatpredicted theoretically. Accordingly, empirical calibration curves canbe constructed that are based on experimental data. Consequently, theCTE versus volume fraction curves are more accurate for manufacturingpurposes than curves obtainable from theoretical relationships such asgiven in Equation 1.

[0076] The graph in FIG. 1 shows two CTE volume fraction curves foraluminum matrix and copper matrix MMC samples prepared with P-120graphite fiber preforms. The aluminum MMC curve was obtained from CTEmeasurements obtained during cooling of samples from 400° C. to 50° C.(800° C. to 50° C. for the copper matrix samples). As shown, P-120reinforced aluminum samples were prepared having fiber volume fractionsin a range of 0.2 to 0.4. The CTE of the resulting samples variedbetween 13 and 4 ppm/°K.

[0077] The degree of deviation from theory is at least in part due to apopulation of fibers with some out of plane orientation. To the extentthat a fiber is oriented out of plane, the elastic restraint of thefiber on the in-plane CTE of the matrix is reduced.

[0078] The microstructure of a sample aluminum alloy MMC with 0.30volume fraction of P-120 fibers is shown in the scanning electronmicrographs of FIGS. 2a and 2 b. The sample was prepared by blending 743grams of DKDX fibers and 119 grams of Carbowax® polyethelyne glycol 8000from Union Carbide Chemical and Plastics Co. (Danbury, Conn.) as abinder.

[0079] The blend was loaded into a 190.5 mm×190.5 mm press die mold andpressed to a thickness of 31.7 mm. Subsequent to heating to atemperature in a range of 80°-100° C. to liquefy the binder, cooling to10° C. solidified the binder to produce a stable, bound preform. Thebound preform was infiltrated with aluminum alloy containing 12.5%silicon and 0.4% magnesium. To obtain these micrographs, polishedsections were taken from the sample MMC along the X-Y plane and alongthe X-Z plane.

[0080] The X-Y section of FIG. 2a shows a substantial number of fiberslaying parallel to the X-Y plane. In the X-Z section of FIG. 2b, only afew fibers lie with any component of their orientation parallel to the Zdirection. Quantitative analysis of several micrographs showed that over84% of the fibers laid within 10° of the X-Y plane, and that over 96% ofthe fibers laid within 30° of the X-Y plane.

[0081] In one embodiment of an MMC of the invention, less than half ofthe fibers are oriented out of the X-Y plane by more than 10°. In a morepreferred embodiment, less than 25% of the fibers are oriented out ofplane by more than 10°. In a more preferred embodiment, less than 20% ofthe fibers are oriented out of plane by more than 10°. In a furtherpreferred embodiment, less than 15% of the fibers are oriented out ofplane by more than 10°.

[0082] Preparation of a Preform

[0083] Properties of an MMC are affected by both the volume fraction andthe orientation of the fibers. For liquid metal infiltrated composites,preferred embodiments use a fibrous preform that does not “swim” orbecome disturbed during the inrush of molten metal. These embodimentsinclude a preform that is stable and that does not lose its shape orfiber distribution during the infiltration process.

[0084] In some embodiments, a stable preform is obtained by denselypacking a preform mold. After removing the packed and bound preform fromthe preform mold, the preform is typically placed into a metalinfiltration mold, in preferred embodiments, a steel can. Preferably,the preform completely fills the metal infiltration mold or the moldcavity of the metal infiltration mold.

[0085] When the preform is heated, the binder may begin to release thefibers. The fibers can then relax and press against the metalinfiltration mold. Some binders evaporate during heating. After thebinder is removed, e.g., with the assistance of an applied vacuum, andthe preform has reached a molten metal infiltration temperature, moltenmetal infiltration can take place.

[0086] In one embodiment, woven fibers in the form of a fabric cloth arecut and loaded directly into a metal infiltration mold for subsequentpressure infiltration. Since a fabric has a discrete thickness,controlling the thickness of an MMC component formed from fabric isdifficult. For volume fractions above the natural woven volume fractionof a fabric, the fabric is compressed and clamped into a mold,increasing tooling costs. Moreover, it is typically difficult to packwoven fabrics to a fiber volume fraction greater than approximately0.45. Conversely, loading molds with fabric to a fiber volume fractionless than 0.40 can lead to non-uniform distribution of the fiber plies.Even when preform cloth plies have been well compressed into a mold,non-uniform ply loading can result in warping after removal from themold.

[0087] An alternative embodiment uses a continuous fiber preform. Such apreform may be fabricated by drum winding continuous fibers, and fixingthe wound fibers onto a transfer sheet by applying a fugitive binder.Such plies can be stacked with orthogonal orientations, or more mixedorientations, for example, including plies oriented at 45° to otherplies. These embodiments can include fiber volume fractions ofapproximately 0.55 to 0.6 or more.

[0088] Another alternative preform material includes a paper-likeproduct produced from chopped discontinuous fibers. The materialincludes a fugitive binder to provide stability and facilitate handling.In a preferred embodiment, the fibers in the “paper” are randomlyorientated in the X-Y plane. This material can be compressed to adesired fiber volume fraction and further stabilized with additionalbinder.

[0089] In another embodiment, a preform of the invention providessubstantially uniform fiber distribution after molten metalinfiltration. The preform is typically prepared by compressing fibersand a binder in a preform mold. In a preferred embodiment, the fibersare discontinuous. The fibers and binder usually are mixed, thencompressed at a pressure that is greater than the molten alloybreakthrough pressure for the finished preform. Prior to metalinfiltration, the binder maintains the compressed configuration of thefibers in the preform so the preform can be removed from the preformmold and placed in a metal infiltration mold. In certain applications, abound preform may be stored for some time prior to metal infiltration.Preferably, the bound preforms are stored below room temperature.

[0090] The binder is often removed from the preform while the preformresides in the metal infiltration mold. Under the constraints of themetal infiltration mold, the preform can maintain its compressed fiberconfiguration. Molten metal is then infiltrated into the preform.Because the fiber configuration remains in its compressed state, it issubstantially undisturbed during infiltration of molten metal at themolten metal breakthrough pressure, and proper in-plane orientation ofthe fibers is maintained.

[0091] In some embodiments, the in-plane distribution of fibers in thepreform mold is enhanced prior to compression and fixation with abinder. For example, the preform mold can be agitated, such as byvibration, prior to compression. Vibration may also break up clumps, or“hair balls”, of fiber. Further, the compression of the fibers and thebinder also serves to enhance the in-plane orientation of the fibers, aswell as increase the volume fraction of fibers in the preform.

[0092] Some embodiments utilize thin sheets of paper-like or felt-likematerial formed from random in-plane oriented fibers. In a preferredembodiment, a sheet is produced with a volume fraction in a range ofapproximately 0.05 to approximately 0.20. The sheets may be weighed toselect a proper amount for a desired preform. The sheets may then beplaced in a preform mold and compressed to obtain a final desired fibervolume fraction in the preform.

[0093] Various binder materials can be employed to form and maintain apreform. A binder material is generally required to maintain fibers in adesired orientation and state of compression. For example, water may bea binder which is set via freezing. Solid forms of polyethylene glycol(“PEG”) may be a binder as well as acrylic. Solid binder materialsusually are heated to approximately or above their melting point, thencooled to solidify. One suitable PEG material is Carbowax® polyethyleneglycol 8000 from Union Carbide Chemical and Plastics Co. (Danbury,Conn.), which is liquefied at approximately 80°-100° C.

[0094] Some binders are fully removed prior to molten metalinfiltration. Other binder materials partially or fully remain duringand after molten metal infiltration. For example, a phenolic-basedbinder may have a volatile component removed prior to molten metalinfiltration. For example, the volatile component leaves in vapor form,leaving behind a carbon-based residue. In some embodiments, non-binderorganic materials may also escape from the preform during evaporation ofsome or all of a binder material.

[0095]FIGS. 3a through 3 e illustrate in cross-section one embodiment ofthe formation of a preform 10. Referring to FIG. 3a, a materialdispenser 22 randomly dispenses discontinuous fibers 11 and a binder 12into a preform mold base portion 20. The binder 12 can be dispensed asdiscontinuous particles as shown, or can be mixed with the fibers byother means. For example, the fibers can be coated with bindersubsequently or prior to distribution of the fibers into the preformmold. One or more dispensers may be employed. In other embodiments,fibers and binder are dispensed from different dispensers.

[0096] Referring to FIG. 3b, to promote a random, in-plane distributionof the fibers, the preform mold base portion 20 often is agitated. Forexample, the base portion 20 can be vibrated after filling with fibers11 and binder 12. Preferably, agitation is applied continuously duringthe dispensing of the fibers and binder into the preform mold.

[0097] Referring to FIG. 3c, a preform mold cap portion 21 is placed incontact with the fibers 11 and binder 12. Pressure is applied via thepreform mold cap portion 21 to the mixture of fibers 11 and binder 12 tocompress the mixture at a pressure greater than the breakthroughpressure required to infiltrate the formed preform with the appropriatemolten metal. The binder 12 serves to fix the configuration of fibersobtained in this compressed state by adhering neighboring fibers 11 toone another to produce the preform 10 as shown in FIG. 3d. FIG. 3e showsthe preform 10 after it is removed from the preform mold base portion20.

[0098] In a preferred embodiment, in a tumble mill to uniformly mix thefibers and binder. The mixture then may be redispersed in a rotary brushmill to untangle the fiber clusters. A measured portion of the mixtureis selected by weighing for production of an MMC of a desired size andfiber volume fraction.

[0099] The weighed portion of fiber and PEG binder is placed in apreform mold base portion. After leveling the mixture by vibration, thepreform mold is closed and the preform is compressed to a predeterminedvolume to obtain the desired volume fraction of fibers in the completepreform.

[0100] When forming a preform from a mixture of PEG powder and fibershaving an average length of approximately 200 μm, pressure ofapproximately 450 psi is required to obtain a fiber volume fraction of30%, while a pressure of approximately 1,100 psi is required to obtain avolume fraction of 40%. These pressures exceed typical liquid metalcapillary breakthrough pressures of the resulting preform.

[0101] Heating to approximately 85° C. melts the PEG. Subsequent coolingallows the PEG to solidify and bind the fibers to one another. Thepreform can then be stored under refrigeration for later use.

[0102] Metal Matrix Materials

[0103] Various metals and metal alloys can be used in the inventiondepending on the particular application. Aluminum, copper and magnesiumare preferred. With applications involving aluminum or an aluminumalloy, it is desirable to add silicon to the metal to reduce thereactivity of the metal with graphite, which undesirably forms aluminumcarbide. For example, an MMC formed from 6061 aluminum alloy with 0.45volume fraction of graphite fibers and had approximately 4.0% carbideformation after pressure infiltration casting. Fabrication of an MMCfrom an aluminum alloy having approximately 7.0% by weight silicon,reduced carbide formation to approximately 0.5%. Using 12.5% silicon inthe aluminum alloy, further reduced the carbide formation toapproximately 0.3%.

[0104] Thus, in embodiments which include aluminum, silicon ispreferably added. In preferred embodiments, the alloy includes at leastapproximately 7% by weight silicon, and more preferably approximately12.5% by weight silicon. The eutectic composition for analuminum-silicon alloy has 12.5% silicon. In addition to reducing theactivity of carbon in aluminum, addition of silicon reduces the meltingpoint of the alloy. This, in turn, further decreases the kinetics ofcarbide formation, provided that metal infiltration temperatures alsoare reduced.

[0105] The matrix alloy should also be able to withstand micro-scaledeformation that can occur during thermal cycling. In some applications,such as integrated circuit heat sinks, an MMC experiences largetemperature cycles during use. Micro-deformation during thermal cyclingcan cause thermal ratcheting, i.e., a change in dimension of the MMCafter each thermal cycle. An accumulation of dimensional changes canlead to damage, for example, of an electronic assembly attached to anMMC heat sink. It is thus desirable to limit deformation over the usetemperature ranges of an MMC, such as from approximately −30° C. toapproximately 150° C.

[0106] To reduce thermal cycling deformation, the metal matrix alloy canbe hardened. In certain embodiments, magnesium is added to aluminum. Incombination with silicon, the magnesium provides an age hardenablealuminum alloy due to formation of precipitates during cooling from, forexample, a brazing temperature. The precipitation hardened matrixsuffers from less plastic deformation during thermal cycling.

[0107] One embodiment of an aluminum-based MMC of the invention includes2.0% by weight or more magnesium in the alloy. A preferred embodimentincludes magnesium in a range of approximately 0.1% to approximately1.0%. More preferred embodiments include magnesium in a range ofapproximately 0.2% to 0.5%, or approximately 0.3% to 0.4%. It should beunderstood that the aluminum alloy can contain minor impurities, forexample, iron, manganese and titanium.

[0108] Other embodiments of the invention use a copper alloy andgraphite fibers. Because the bond strength between copper and graphiteis extremely low, chromium is often included in the alloy. The chromiumreacts with carbon in the fibers to form chromium carbide at thefiber-metal interface, which aids the bonding between the alloy and thefibers.

[0109] One embodiment includes approximately 5.0% by weight or morechromium in the alloy. A preferred embodiment includes chromium in arange of approximately 0.3% to 5.0%. A more preferred embodimentincludes chromium in a range of approximately 0.3% to 2.0%. Other morepreferred embodiments include chromium in a range of approximately 0.5%to 1.5%, or approximately 0.7% to 1.0%.

[0110] Another embodiment includes a copper alloy having improved yieldstrength through addition of zirconium. Zirconium promotes solidsolution hardening and reduces thermal ratcheting. A preferredembodiment includes zirconium in a range of approximately 0.1% to 2.0%by weight. A more preferred embodiment includes zirconium in a range ofapproximately 0.1% to 1.0%. Other more preferred embodiments includezirconium in a range of approximately 0.1% to 0.5%, or approximately0.12% to 0.3%. Alloy additions, such as those described above, have aminimal effect on the thermal conductivity of an alloy.

[0111] Infiltration

[0112] Using methods known in the prior art, it is typically difficultto control MMC volume fraction by stacking woven cloth preforms into amold. Since graphite fibers are not easily wetted by some alloys, suchas aluminum, magnesium and copper, pressure infiltration is used toovercome the wettability difficulty. However, a pressure infiltratedmetal exerts a compressive pressure on a preform prior to capillarybreakthrough and subsequent infiltration. In some cases, theinfiltrating metal compresses the preform and causes veining and grossdisplacement of the preform to the mold wall upon breakthrough into lessdense regions. Obtaining a particular desired volume fraction of fibersis also difficult when using stacked wrapped lamina to make a preform.

[0113] As described above, methods of the invention overcome thesedifficulties by providing a preform which has been compressed at apressure above that experienced during infiltration casting. Forexample, by pressing preforms to a pressure greater than the capillarybreakthrough pressure, activating a binder to constrain the preform,loading the fiber and binder into a fixed volume mold, removing thebinder by evacuation and heating just prior to pressure infiltrationcasting, a metal matrix composite can be manufactured free ofbreakthrough defects and at a controlled volume fraction reinforcement.

[0114] Preforms of the invention can be infiltrated individually orcollectively. FIG. 4 illustrates an embodiment of multiple preformsprepared for infiltration. The preforms 10 are layered with separatorsheets 15, for example, graphite foil sheets, to permit production ofmore than one MMC component during one infiltration cycle. The separatorsheets may also be, e.g., slices of graphite, graphite coated steelsheets, or colloidal graphite coated sheets. The separator sheets easeseparation of the MMC components after cooling of the metal.

[0115] In another embodiment, as illustrated in FIG. 5a, one or morepreforms 10 are placed adjacent to each other without separator sheets.Hence, surfaces of the preforms 10 are in direct contact. After packingthe layered preforms 10 into a metal infiltration mold vessel, thebinder is removed, for example, by heating. As illustrated in FIG. 5b,upon release of the binder, the contacted surfaces of the preforms 10can merge with one another and permit formation of an effectively largerpreform and ultimately larger MMC component.

[0116] A preform or stack of preforms can be infiltrated with a moltenmetal by any method known to one skilled in the art. In one embodiment,as illustrated in FIG. 6, a stack of preforms 10 layered with separatorsheets 15 is placed in a metal infiltration vessel 30. A filter 33 isplaced on top of the stack to prevent premature infiltration of thepreform, especially if the preform is evacuated prior to introduction ofthe metal. Note, however, that alternative arrangements of preforms inmold vessels are possible which may not required a filter, for example,use of gated top plates or caps.

[0117] In another embodiment, illustrated in FIG. 7, a preform 10 ishorizontally positioned in a metal infiltration vessel 30. A cap 33 withgates 39, for admission of molten metal, is placed on the preform 10.The cap may be held in place by means known in the art which includeswelding. In embodiments where a high volume fraction of fibers isdesired, the preform(s) typically need to be isolated in a confinedspace so that upon removal of the binder, the fibers maintain theirposition, orientation and compactness.

[0118] One can employ a mold release agent in the vessel. For aluminumalloy and magnesium alloy, the mold release agent preferably is one ormore layers of colloidal carbon, e.g., colloidal graphite or boronnitride, which is dispersed in a suitable volatile vehicle. However,other ceramic slurry coatings may be used. For copper alloy, a slurry ofzirconium oxide in a slightly acidic vehicle sold under the trade nameZircwash™ may be used. Other parting compounds may be used as moldrelease agents or washes such as boron nitride or graphite foil.

[0119] In one embodiment, a preform is tightly loaded into a moltenmetal infiltration vessel. The preform is heated to remove binder viaevaporation. Upon removal of the binder, compressive stresses stored inthe preform cause the preform to relax against the walls of the vessel.Since the preform is constrained by the walls of the vessel, the vesselwalls now maintain a compressive stress on the preform that is greaterthan the breakthrough pressure of the molten metal.

[0120] In a preferred embodiment, the process described in U.S. Pat. No.6,148,899 is used to infiltrate molten metal into a preform. Briefly,liquid metal is transferred by vacuum siphon into the metal infiltrationmold vessel which is under reduced pressure. The mold vessel is placedin an autoclave and pressurized to approximately 60 atm using nitrogengas, forcing the molten metal into the preform.

[0121] The mold vessel then is contacted with tin-bismuth at theeutectic composition. Heat from the vessel causes the tin-bismuth tomelt. This heat transfer process increases the solidification rate ofthe molten metal in the preform and assists directional solidificationto help eliminate shrinkage porosity.

[0122] After cooling, the MMC component is removed from the vessel. TheMMC component can than receive other processing, for example, machininginto a final desired shape for use as a heat sink, or plating inpreparation for some types of brazing.

[0123] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Theforegoing embodiments are therefore to be considered in all respectsillustrative rather than limiting on the invention described herein.Scope of the invention is thus indicated by the appended claims ratherthan by the foregoing description, and all changes which come within themeaning and range of equivalency of the claims are intended to beembraced therein.

[0124] Each of the patent documents and scientific publicationsdisclosed hereinabove is incorporated by reference herein.

What is claimed is:
 1. A metal matrix composite comprising: a metalalloy; and random in-plane discontinuous fibers, wherein the randomin-plane discontinuous fibers comprise carbon.
 2. The metal matrixcomposite of claim 1 wherein the metal alloy comprises a major componentselected from the group consisting of aluminum, copper, and magnesium.3. The metal matrix composite of claim 1 wherein a majority of therandom in-plane discontinuous fibers have a length less thanapproximately 750 μm.
 4. The metal matrix composite of claim 1 whereinthe random in-plane discontinuous fibers comprise graphite.
 5. The metalmatrix composite of claim 1 wherein the random in-plane discontinuousfibers are milled.
 6. The metal matrix composite of claim 1 wherein themetal matrix composite has a volume fraction of the random in-planediscontinuous fibers in a range of approximately 0.15 to approximately0.6.
 7. The metal matrix composite of claim 1 wherein a minority of therandom in-plane discontinuous fibers are oriented out of plane by anangle greater than 10°.
 8. The metal matrix composite of claim 7 whereinless than 20% of the random in-plane discontinuous fibers are orientedout of plane by an angle greater than 10°.
 9. The metal matrix compositeof claim 1 wherein an in-plane coefficient of thermal expansion is in arange of approximately 3 ppm/°K to approximately 12 ppm/°K.
 10. Themetal matrix composite of claim 1 wherein an in-plane coefficient ofthermal expansion is greater than the coefficient of thermal expansionof silicon.
 11. The metal matrix composite of claim 2 wherein the majorcomponent of the metal alloy is aluminum, and the metal alloy furthercomprises more than approximately 4 wt % silicon.
 12. The metal matrixcomposite of claim 11 wherein the silicon composition is approximatelythe eutectic composition.
 13. The metal matrix composite of claim 1wherein the random in-plane discontinuous fibers are uniformlydistributed within the metal matrix composite.
 14. The metal matrixcomposite of claim 11 wherein the metal alloy further comprises a minorcomponent that enables precipitation hardening.
 15. The metal matrixcomposite of claim 14 wherein the minor component is less thanapproximately 2 wt % magnesium.
 16. The metal matrix composite of claim2 wherein the major component of the metal alloy is copper, and themetal alloy further comprises less than approximately 5 wt % chromium.17. The metal matrix composite of claim 16 wherein the metal alloyfurther comprises a minor component that enables solution hardening. 18.The metal matrix composite of claim 17 wherein the minor component is atless than approximately 2 wt % zirconium.
 19. The metal matrix compositeof claim 18 further comprising a nickel plating.
 20. An article ofmanufacture comprising the metal matrix composite of claim
 1. 21. Ametal matrix composite comprising: a metal alloy; and random in-planediscontinuous fibers, wherein the random in-plane discontinuous fiberscomprise carbon and are uniformly distributed within the metal matrixcomposite, and wherein the metal matrix composite has a volume fractionof the random in-plane discontinuous fibers in a range of approximately0.15 to approximately 0.6.
 22. A metal matrix composite comprising: ametal alloy consisting essentially of aluminum, silicon and magnesium,wherein the silicon is approximately 5 wt % to approximately 20 wt % ofthe metal alloy, and the magnesium is approximately 0.1 wt % toapproximately 2 wt % of the metal alloy; and random in-planediscontinuous graphite fibers uniformly distributed within the metalmatrix composite.
 23. A metal matrix composite comprising: a metal alloyconsisting essentially of copper, chromium and zirconium, wherein thechromium is approximately 0.3 wt % to approximately 2 wt % of the metalalloy, and the zirconium is approximately 0.1 wt % to approximately 1 wt% of the metal alloy; and random in-plane discontinuous graphite fibersuniformly distributed in the metal matrix composite.
 24. A method ofmanufacturing a metal matrix composite, the method comprising the stepsof: contacting random in-plane discontinuous fibers with a binder;pressurizing the random in-plane discontinuous fibers and the binder toform a bound preform, wherein the random in-plane discontinuous fibersand the binder are pressurized to a pressure greater than the capillarybreakthrough pressure of the bound preform; placing the bound preform ina mold; infiltrating the bound preform with a molten infiltrant under apressure at least equal to the capillary breakthrough pressure; andcooling the molten infiltrant to form the metal matrix composite. 25.The method of claim 24 further comprising the steps of: placing a secondbound preform adjacent to the bound preform in the mold prior to thestep of infiltrating; contacting a surface of the bound preform with asurface of the second bound preform; and removing the binder prior tothe step of infiltrating to merge the surface of the bound preform withthe surface of the second bound preform.
 26. The method of claim 24further comprising the steps of: heating the bound preform in the mold;evacuating the bound preform in the mold to create a reduced pressurewithin the bound preform; and transporting a charge of the molteninfiltrant into the mold while maintaining the reduced pressure withinthe preform.
 27. The method of claim 24 further comprising the step of:forming a preform of random in-plane discontinuous fibers, wherein thestep of forming the preform comprises agitating discontinuous fibers topromote a random in-plane orientation.
 28. The method of claim 24wherein the binder comprises a particulate, and the method furthercomprises the steps of: liquefying the binder; and solidifying thebinder to form the bound preform.
 29. The method of claim 24 wherein therandom in-plane discontinuous fibers comprise carbon.